I think part of the reason is they wanted to send a message. Not the one you'd think: "Don't terrorize your neighbors" or "Don't threaten politicians". The message is "You'd better take the plea bargain. Or else." Prosecutors don't have time to take every case to trial, so they threaten everybody with 100 little charges that add up to 150 years in jail. Then they offer you a deal, where you plead guilt to some subset and spend enough years in jail to satisfy the government.
It's grossly unfair. Nobody knows how many innocent people are wasting away in prison because they didn't want to risk spending the rest of their lives there.